Leak testing of sealed packages is desirable to insure the integrity of the seal. Where the package is a sealed can having a metal end, it is known from U.S. Pat. No. 2,093,429 to confine the metal end by holding it in a testing head and removing atmospheric pressure from the outside face of the end. The end may flex outwardly at such time which is indicative of a leak or where the vacuum still obtains in the sealed can there will be no movement of the end. However, this known method and the testing machine for practicing it are not applicable to packages having other flexible wall portions since the removal of atmospheric pressure from outside the face of one end of the package would collapse the package.
U.S. Pat. No. 3,371,781 discloses an apparatus for inspecting sealed containers at high production rates to determine whether or not an adequate vacuum exists within the container. A proximity sensor is mounted to be accurately positioned with respect to a flip panel in the container cap, regardless of variations in container height or cocking or tilting of the cap on the container. This conventional method and apparatus are useful for inspecting sealed containers such as glass jars with metal caps for baby food where the caps for the jars have a flip panel. However, this passive method and the related apparatus are not useful for leak testing sealed packages such as flexible or semi-flexible packages which do not include a flip panel.
One recent attempt to overcome the aforementioned limitations of known leak testing methods and apparatus in testing flexible packages is disclosed in U.S. Pat. No. 4,774,830. The patent discloses a leak detector for detecting leaks in flange-shaped seals between lid and body portions of a package wherein a pressure chamber is provided which isolates the external edge of the seal and applies test pressure thereto. Containment of the pressure chamber includes sealing pressure applied mechanically to the flange-shaped package seal during testing, but this sealing pressure does not affect seal testing because it is appropriately limited by a self-compensating biasing of the mechanical seal. One disadvantage of this known method and apparatus is that it is limited to detecting defects in the seal itself. Defects adjacent the seal, such as stress cracks in the package near the seal, are not detected in this known method and apparatus.
It is also known to test sealed flexible containers by placing them in an evacuated chamber and detecting the degree to which the walls of the container bulge in this condition since the pressure is greater inside the container than outside the container within the chamber. This "bulging wall test" is problematical however, especially in the case of containers which are filled with no head space, since the contents of the container can plug a leak and make it difficult to reliably detect a defective container. A recent effort to solve these problems is disclosed in U.S. Pat. No. 4,901,558 wherein a microprocessor is used to simultaneously examine the measured deflection of the walls or sides of sealed packages placed in a vacuum chamber and evaluate the status of the vacuum within the vacuum chamber.
It is also known to apply a compressive force to a package and observe the reaction of the package for purposes of detecting a leak. For example, in U.S. Pat. No. 4,930,345, the apparatus inwardly elastically deflects the walls of a sealed cup and monitors the bulged position of a flexible cover on the cup U.S. Pat. No. 4,747,299 discloses a method of testing a package seal wherein the pressure on the outside of a cup of the package is reduced to bow the flexible lid inwardly. A change in the position of the distended wall of the cup is sensed for determining the integrity of a package seal. U.S. Pat. No. 4,899,574 is directed to a method and apparatus for detecting leaks in a sealed container wherein a compressive force is applied to a flexible lid of the container so as to circumscribe an area of the lid which is coaxial with the center of the cup. The circumscribed area bulges outwardly and the position thereof is detected. During testing, the sealed container is gripped between upper and lower test assemblies which engage opposite sides of the seal at the container lip to form respective chambers about the opposite sides of the container. Pressure or vacuum conditions can be maintained within these chambers during testing. Each of these known methods and apparatus require the application of compressive force to the sealed package and at least the apparatus and method of U.S. Pat. No. 4,899,574 may not successfully detect defects located in wall portions clamped between the upper and lower test assemblies.
There is a need for an improved method and apparatus for leak testing sealed packages, particularly flexible packages, which do not suffer from the aforementioned limitations and drawbacks of the known leak testing methods and apparatus.